1. Field of the Invention
The present invention relates magnetic recording media used for the storage of data information. It relates more particularly to thin film magnetic recording media using aluminum alloy disk substrate.
2. Description of the Prior Art
As the typical examples of such media in the prior art, there can be mentioned those disclosed in Japanese Patent Publication Sho 48-18842 and Japanese Patent Laid-open Applications Sho 58-51024 and Sho 60-261022.
FIGS. 10(a), (b) and (c) show, respectively, layered structures of typical magnetic disks put to practical use in the prior art. Layers of ferromagnetic metal or oxide are formed on a substrate of non-magnetic Al-Mg alloy by means of electroless plating or sputtering. FIG. 10(a) shows a plated disk prepared by forming a Ni-P undercoat (2) on a substrate (1) of Al-Mg alloy by means of electroless plating to a thickness of from 10 to 30 .mu.m for improving the precious fabricability at the surface of the substrate, which is then finished into a smooth surface and a magnetic layer (3) on which magnetic information is actually recorded is formed from ferromagnetic material of Ni-Co-P alloy by plating on the thus finished substrate. Then a surface protective layer (4) such as of SiO.sub.2 is further formed. Values for the thickness of respective layers are shown for the reference in the brackets.
FIG. 10(b), shows a structure in which a ferromagnetic Ni-Co alloy material is formed by sputtering on a substrate with a Ni-P undercoat (2) thereby form a magnetic layer (3'). A Cr underlayer (5) is formed between the magnetic layer (3') and the Ni-P undercoat (2), while a protective carbon layer (4') is formed at the surface. FIG. 10(c) shows a structure in which an Al.sub.2 O.sub.3 layer (2') is formed instead of the Ni-P undercoat on a substrate (1) by using alumite process and a magnetic layer (3") made of ferromagnetic .gamma.-Fe.sub.2 O.sub.3 material is formed thereover.
In the magnetic recording media of this type, since the thickness of each of the magnetic layers (3), (3') and (3") is thin, adhesion thereof with the substrate or the hardness of the substrate restricts the abrasion resistance. In order to provide magnetic recording media with various characteristics, it is at first required that the substrate can satisfy the following conditions, that is,
(I) has high degree of surface finish (surface roughness Ra&lt;0.01 .mu.m), PA1 (II) has no such head hit protrusions at all as hindering the stable flying of a magnetic transdusing head, PA1 (III) has little defects which may cause signal errors or no such defects at all, and PA1 (IV) has a sufficient hardness not to suffer from deformation damages upon contact-start-stop with the head.
For the required characteristic (IV), since the surface protective layer (4) or a lubricant layer also has a close relation therewith, actual conditions only with respect to the substrate are not clear. However, for other required characteristics, since the thickness of the magnetic layer has to be reduced in view of the information recording density, the drawback or the incomplete nature of the substrate can not be compensated by the upper layers.
In order to cope with such problems, substrates prepared by applying the Ni-P undercoat (2) to a thickness of 10-30 .mu.m by means of electroless plating or applying the Al.sub.2 O.sub.3 layer (2') on a non-magnetic substrate made of Al-Mg alloy and subsequently applying surface finishing by means of polishing has been used, and a magnetic layer is formed further thereover in the prior art. That is, the magnetic layer has not been formed directly on the surface of the substrate (1) of Al-Mg alloy in the prior art. Such direct formation can not be put to practical use, because there has been known no appropriate method for making the surface of the Al-Mg alloy substrate free from protrusions in the prior art and because a lot of intermetallic compounds such as Al-Fe and Mg-Si are present at the surface of the Al-Mg alloy substrate, which deteriorate the characteristics of the magnetic layer (3) in view of various points.
However, the Ni-P undercoat (2) or Al.sub.2 O.sub.3 layer (2') is not necessary in the magnetic recording and additional formation of such layer increases the cost of the magnetic recording media. Particularly, since Ni is generally expensive material, it results in a further increase in the cost.